Understanding Magnetic Flux Emergence

Understanding Magnetic Flux Emergence

The strong toroidal magnetic field produced by rotational shear at
the base of the convection zone is thought to be the source of buoyant
flux loops that rise and emerge to form magnetic active regions on the
solar surface. Subsequently, the emerged field evolves in the
photosphere, chromosphere, and corona where they play a critical role in
structuring and forcing into the atmospheric layers. Observational,
theoretical, and numerical investigations that HAO scientists have
undertaken provide a vital understanding of how the precursor structures
for space‐weather events such as flares and coronal mass ejections
(CMEs) form in the outer atmosphere as well as of the steady mass and
energy transport originating from the base of the atmosphere. The main
objective is to understand the emergence, growth, decay, and
distribution of the surface fields on all spatial scales, their
transport by diffusion and flows, the effects of the flux eruption on
the surrounding and overlying atmospheric regions, and their eventual
contributions to solar irradiance variability.

HAO scientists are making progress towards three‐dimensional global
magneto‐hydrodynamic (MHD) simulations in which turbulent convection,
stratification, and rotation are combined to yield a dynamo that
self‐consistently generates buoyant magnetic loops for the Sun and
Sun‐like stars. Recent steps in this direction involve simulating
convection and dynamo action in a fast rotating spherical shell with
solar stratification. The simulations of fast‐rotating Sun‐like stars
have demonstrated that strong “wreaths” of toroidal magnetic field are
formed by dynamo action in the convection zone, and that the strongest
portions of those wreaths will rise to the top of the convection zone
via a combination of magnetic buoyancy instabilities and advection by
giant convective cells. Conversely, for the slower solar case, the
effects of the convective flow will allow the inserted flux tubes with
weaker field strengths (below 50 kG at the bottom of the solar
convection zone) to develop emerging loops. Those loops have properties
that are consistent with those observed in solar active regions.

The solar photosphere is a transition region in which the primary
energy transport mechanism switches from convection to radiative
transfer. At the same time, the plasma becomes partially ionized due to
the lower temperatures, requiring a more complicated equation of state.
The role of the magnetic field is changing too: while the gas pressure
dominates the interior of the Sun, the magnetic pressure becomes the
dominant contributor in or above the photosphere. Owing to the rather
short density scale height, the photosphere is a highly stratified
medium in which convective motions easily steepen to supersonic flows
and shock waves. The combination of all these conditions makes numerical
modeling of the photosphere challenging but also extremely interesting
due to the strong interaction between convection, magnetic field, and
radiation, and the possibility for in‐depth comparison with
high‐resolution observations.

Figure 2.Sunspot
fine structure at the τ = 1 level. Quantities shown are (a) bolometric
intensity, (b) radial and (c) vertical magnetic field, (d) field inclination,
(e) radial, and (f) vertical flow velocities. A field inclination of 0°
corresponds to vertical field with the same polarity as the umbra, 90° to
horizontal, and 180° to vertical field with opposite polarity of the umbra.
Radial outflows are displayed by red colors, solid contours indicate regions
with more than 10 km s–1 outflow velocity. Vertical upflows are
displayed by blue colors, solid contours indicate regions with more than 5 km s–1
downflow velocity.

By studying the properties of sunspots
HAO scientists are learning about the complexities of the photospheric
interface. High-resolution simulations are carried out to investigate the
connection between the fine structures of sunspot penumbrae and the subsurface
magneto-convection processes that are responsive for filamentation of the
penumbra and acceleration of the eponymous Evershed outward flow of materials.
Simulations at lower resolutions but on larger computational domains (up to 16
Mm deep and 75 Mm wide) are also performed to study the subsurface magnetic and
flow structure of sunspots, including large scale-flows in the surrounding moat
region. The sunspot fine structures are illustrated in Figure 2. Developed in
partnership with the NASA Solar Dynamo Observatory (SDO) Science Center at the
Colorado Research Associate (CoRA), these simulations have permitted the critical
validation of helioseismic structural inversions of sunspots and their
surroundings.

The diagnosis of the prevailing magnetic
conditions in the lower solar atmosphere is conducted by measuring the
polarization of light emitted by magnetically sensitive spectral lines.
Historically, HAO has pioneered the development and use of instruments to make
such spectro-polarimetric measurements, along with techniques to infer field
strength and direction from those observations. Solar physics is entering a new
era of magnetic field measurements, in which emerging technologies and
observations from space will revolutionize our overall understanding of the
Sun’s magnetism and its impact throughout the heliosphere. At the core of these
investigations are the observations and detailed analysis of magnetic features
as they emerge through the solar photosphere. Through involvement with the
spectro-polarimeter (SP) portion of the Hinode spacecraft’s solar optical
telescope (SOT) instrumentation package, HAO scientists are able to study in
detail magnetic flux emergence, decay, and distribution, at a range of spatial
scales.

At the smallest scales, the atmosphere of the
quiet sun is pervaded by an apparently turbulent magnetic field with various scale
sizes extending well below the resolution of present-day instruments. The
small-scale structure of solar magnetism might arise from disruption and
shredding through the action of convective flows on the field of the
large-scale solar magnetic cycle as a result of a dynamo operating at the base
of the solar convection zone. Alternatively, it might arise from a small-scale
turbulent dynamo (SSD) operating as a result of vigorous turbulent convective
motions very close to the solar surface. It is important to distinguish between
these two possibilities because the fields arising from a small-scale dynamo
may affect the structure of the solar atmosphere independent of the solar
magnetic cycle. The SSD mechanism has been suggested by numerical magneto-convection
simulations, but so far there is little observational evidence confirming or refuting
its presence. The high-resolution and excellent polarimetric sensitivity of the
Hinode/SP has permitted observations that suggest the SSD indeed is operating
in the solar atmosphere through the detailed examination of very quiet regions.